Auto hammers are commonly used as portable tools in decoration and construction fields. Chinese Patent Application No. 200820161342.1 discloses an auto hammer which comprises a main body and a muzzle part connected to the main body. The muzzle part is generally composed of a hollow cylindrical sleeve in which a hole for receiving a magnet is drilled. The magnet is engaged in the hole for attaching a nail placed in a striking device to clamp the nail. The disadvantages of such auto hammer are that: the magnet is located at the edge of the sleeve, so that the nail cannot be positioned at the centre of the sleeve and cannot be parallel to the centre line of the sleeve (i.e., the attached nail is deflected). The magnet also cannot clamp other non-magnetic materials, such as wooden tenons and the like.